Purple sweet potato has recently received attention as a healthful food. Purple sweet potato (PSP) is rich in anthocyanins and phenolic compounds. Anthocyanins are the deeply pigmented red, purple and blue flavonoid compounds that have significant physiological effects and bioactivity in humans. Anthocyanins are well known from medical research for their capillary health stabilizing effects, increasing the strength of the capillary wall to help reduce bleeding. They are part of the vitamin P complex of bioflavonoids which is related to rutin and citrus bioflavonoids.
One of the earliest studied sources of anthocyanins has been from Bilberry (Vaccinium myrtillus) or European Blueberry which is a smaller, wild type of low bush blueberry that has a long history of use in European countries for improving eye health and night vision. Some earlier studies using much higher dosages of anthocyanins showed promising effects for myopia and night vision. More recent studies with anthocyanins in the oligomeric form derived from grapes have shown a beneficial effect for myopia and night vision at lower to moderate dosages (Lee J, 2005), and for video terminal eye fatigue (Nakaishi H, 2000).
Purple sweet potato and its anthocyanins have shown similar brain and cognitive effects as blueberries in both in vitro and in vivo studies (Wu D M, 2008; Ye J, 2009; Shan Q, 2009; Wang Y J 2009; Park 2010). The extract inhibited lipid peroxidation from ferrous ion and ascorbic acid in rat brain homogenates (Cho, 2003) and strongly improved cognitive performance, assessed by passive avoidance test in ethanol treated mice.
Purple sweet potato has been shown to repair d-galactose-induced spatial learning and memory impairment by regulating the expression of synaptic proteins. The effect of purple sweet potato on the spatial learning and memory has been demonstrated by studies in vivo in a water maze with mice treated with d-galactose. Memory formation depends on pre- and post-synaptic proteins, but low levels can be improved by treatment with purple sweet potato extract (Wu D M, 2008).
Anthocyanins from purple sweet potato suppress the development of atherosclerotic lesions. Vascular damage and insufficiency is also implicated in cerebrovascular insufficiency, stroke and aging brain pathologies. Studies have shown there are protective effects of anthocyanins from purple sweet potato against low-density lipoprotein (LDL) oxidation and atherosclerotic lesion development in mice (Kano M, 2005; Miyazaki K, 2008). It protected LDL against oxidation more effectively than other anthocyanins and 1-ascorbic acid in vitro. Atherosclerotic plaque area was reduced to about half of controls, oxidative stress markers, and soluble vascular cell adhesion molecule-1 were also reduced (Miyazaki K, 2008).
Amyloid-beta peptide is implicated in Alzheimer's disease pathology and is known to induce redox imbalance, mitochondrial dysfunction and caspase activation, resulting in neuronal cell death. An in vitro study of purple sweet potato anthocyanins decreased the toxicity of amyloid peptide free radicals, lipid peroxidation, and cell death which may help prevent loss of cognitive function, help sustain energy production in mitochondria, and reduce risk of dementia (Ye, 2010). Purple sweet potato color ameliorates cognition deficits and attenuates oxidative damage and inflammation in aging and the anthocyanins improved learning and memory ability, suppressed cyclooxygenase-2 (COX-2) and inducible nitric oxide synthases (iNOS) expression in mouse brain, decreased excess overproduction of tumor necrosis factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta) and interleukin-6 (IL-6) in the mouse brain. By decreasing pro-inflammatory pathways purple sweet potato color suppresses lipopolysaccharide-induced acute inflammatory responses in the brain (Wang Y J, 2009).
Another area of health benefits of purple sweet potato is its ability to reduce damage to blood vessels by excess sugar in the blood which causes protein glycation, also a known factor in beta amyloid synthesis (Park, 2010). Blood pressure reducing effects can also decrease damage to the brain. It has been shown that purple sweet potato has anti-hypertensive effects (Shindo, 2007).
Purple sweet potato has long been staple food in China, Japan, and Korea because of its taste and its health benefits. Purple sweet potato can be prepared in many creative ways for both sweet and savory taste. For example, purple sweet potato can be steamed, boiled, baked, stewed, and made into dumplings. In the West, purple sweet potato finds its niche. Many lifestyle and health care experts such as livestrong.com and Dr. Oz, highly recommend consuming purple sweet potato. Markets for purple sweet potato chips and purple sweet potato pies are emerging.
In order to reap its maximum health benefit, one has to consume a large quantity of purple sweet potato on a regular base. Unfortunately, purple sweet potato is still considered to be a novelty item. It is not always available in the general markets. Very often one has to search for them in Asian groceries. Furthermore, purple sweet potato is a flatulence-producing food because it is high in certain polysaccharides, especially oligosaccharides such as inulin. For people who are prone to bloating and suffer from flatulence, purple sweet potato may make the problem worse. The purple sweet potato liquid extract produced with this invention contains as much as 500 mg of anthocyanins and as much as 6,155 mg total phenolic compounds per 100 gm of the liquid, which is equivalent to consuming a 100 gm of baked purple sweet potato1 without the starch.
Purple sweet potato is heavily used in the industry to provide natural pigments alternative to synthetic colorants. The process to extract PSP powders is designed primarily to obtain high purity and high color anthocyanin pigments. However, the process is complex and costly. A large amount of anthocyanins, phenolic compounds and other active ingredients are also lost in the process. The process also creates environmental pollution because it uses citric acid or hydrochloric acid solution and acidified ethanol solution as the extraction solvent to extract the anthocyanin pigments. In summary, existing process suffers from the low extraction rate of anthocyanins, high production cost, loss of nutritional content and creating waste and pollutant.
The purpose of the present invention is to overcome the deficiencies of the existing manufacturing process and to produce a natural purple sweet potato juice that is free from starch, rich in anthocyanins, phenolic compounds, and soluble dietary fiber with low viscosity, attractive color, and with natural sweetness.